1. Field of the Invention
This invention relates generally to telemetry devices and more particularly to telemetry devices disposed on or within a pneumatic wheel assembly.
2. Description of the Prior Art
The prior art teaches the concept of disposing a telemetry device within a pneumatic wheel assembly. Usually the telemetry device is used to signal when the pneumatic tire of the wheel assembly has gone flat, but may also signal such information as tire pressure, tire temperature, etc.
For example, in U.S. Pat. No. 4,237,728 of Betts a run-flat sensing system is described. Betts' system includes a sending unit `A` attached within each wheel assembly of a vehicle, and a receiving unit `B` attached to the frame of the vehicle. When a tire goes flat, a piezoelectric material 14 generates electricity to power a transmitter.
In U.S. Pat. No. 4,117,452 of Snyder a telemetry device is described which has a piezoelectric crystal that is deformed by a mechanical member 64 when the tire is flat. Mechanical member 64 is permanently extended into the void of the tire.
The wheel of a pneumatic wheel assembly has a pair of rim lips flanking a circumferential depression. The logical place to locate a telemetry device is within the circumferential depression but, since the wheel is invariably metallic, it is very difficult to transmit a strong signal from that location, due to the R. F. shielding effect of conductors. The prior art has addressed this problem by making the transmitter of the telemetry device very powerful so that a detectable signal can reach the receiver of the telemetry device. The prior art transmitters are heavy (which tends to unbalance the wheel assembly), and require a great deal of power.
A partial solution to the signal attenuation problem would be to permanently extend an antenna from the transmitter of the telemetry device into the void of the pneumatic tube and transmit through the substantially non-metallic sidewalls of the tire. One problem with such a solution is that it would be very difficult to remove and mount the tire because the antenna would get in the way. Another problem with this solution is that the antenna, transmitter, and tire could possibly be damaged when the tire goes flat.